The present invention generally relates to fluid sensing devices and process for producing such devices. More particularly, this invention relates to a micromachined fluid sensing device capable of measuring properties such as fluid density, specific gravity, and chemical concentrations, and in which a fluid bypass is integrated into the device to enable use of the device in flow systems that exceed the internal flow capacity of the device.
Processes and designs for resonant mass flow and density sensors using silicon micromachining techniques are disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al. and U.S. Pat. No. 6,647,778 to Sparks. As used herein, micromachining is a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer), or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. In the processes disclosed by Tadigadapa et al. and Sparks, wafer bonding and silicon etching techniques are used to produce microelectromechanical systems (MEMS) comprising one or more suspended silicon tubes on a wafer. The tube is vibrated at resonance, by which the flow rate and density of a fluid flowing through the tube can be determined.
Sensors of the type taught by Tadigadapa et al. and Sparks have found use in a variety of applications. A notable advantage of these sensors is the extremely miniaturized scale to which they can be fabricated, which among other things enables the sensors to precisely analyze very small quantities of fluids. However, in certain applications where relatively large volume flow rates exist, the limited flow capacity of these miniaturized sensors can be inadequate. It would be advantageous if sensors of the type taught by Tadigadapa et al. and Sparks could be adapted for relatively high flow applications without necessitating an increase in sensor size.